Question: In chemistry class, the girl to boy ratio is $9$ to $2$. If there are a total of $22$ students, how many boys are there?
Explanation: A ratio of $9$ girls to $2$ boys means that a set of $11$ students will have $9$ girls and $2$ boys. A class of $22$ students has $2$ sets of $11$ students. Because we know that there are $2$ boys in each set of $11$ students, the class must have $2$ groups of $2$ boys each. There is a total of $4$ boys in chemistry class.